1. Field of the Invention
This invention relates to optical beam splitters for separating three colored light beams from a light beam having the image content of an original.
2. Description of the Prior Art
In image analyzing devices, it is conventional to scan, point-to-point, an original with a spot of light and to analyze the modulated light transmitted by or reflected from the original to determine its spectral content and brightness. The analysis is generally effected by splitting the modulated light into three colors, for example red, green and blue and to measure the amount of light of each of the colors by means of respective photoresponsive devices. The scanning can be effected by means of a raster on the face plate of a cathode ray tube which is imaged onto the original. Alternatively, the point of light may be caused to move across the original respectively in one direction while the original is moved (at a slower rate) orthogonally in its own plane.
The modulated and split light is collected and directed onto the respective photoresponsive devices and signals therefrom are representative of the color content of the original on a point-to-point basis. Such signals can be stored and/or used indirectly or directly in a reproductive process.
The beam splitting may be effected by semireflective mirrors which may be dichroic and/or which may be followed by color filters so that each photoresponsive device receives only light of one of the three colors. A conventional beam splitter is in the form shown in FIG. 1 of the accompanying drawings and comprises three prisms, two of which have dichroic filters coated on one of their faces. Such a beam splitter is costly and has inherent disadvantages but is of the kind normally used when two dimensional scanning of an original is effected such as in a television camera.
U.S. Pat. No. 2,792,740 describes a more basic form of beam splitter but explains that problems arise using inclined dichroic or semitransparent mirrors as the beam splitting elements. These problems are: "ghost images" due to interreflections between the surface of each of the mirror supports, transverse chromatic aberration, coma and astigmatism.
Linear charge coupled devices (CCDs) have now been developed. The individual cells of the array are small enough to give a pixel size providing acceptable resolution. An illuminated line of an original to be scanned is imaged on the array. Three such arrays, with an appropriate beam splitter are used. The original is preferably moved so that successive lines thereof are imaged on the arrays. A linear CCD array may typically be 10 to 15 mm long and contain 1000 cells in the linear array. Each pixel is then 1 thousandth of the width of the original in the direction of the line of illumination. For example, the light received by each cell of the CCD, for acceptable definition, derives from an area of the original no greater than 0.25 mm (and preferably of the order of 0.025 mm) length in the direction of the line of illumination.
Unfortunately, the amount of light received by each cell (the integral of light against time) is very small and the lens, imaging the line of light on the array, to provide a sufficient quantity of illumination, must therefore work at an aperture of, for example, f4 or greater. The greater the aperture of the lens, the greater is the tendency for optical aberrations and other optical faults to arise. When using linear CCD arrays, as described above, faults such as astigmatism and ghost images which would be present using the arrangement described in U.S. Pat. No. 2,792,740 would be totally unacceptable. The arrangement described in that specification will overcome the problem of coma and transverse chromatic aberration and normalizes astigmatism for all three channels. Astigmatism may also be overcome totally but in a way that creates ghost images transverse to the line of illumination. The problems of ghost images and/or astigmatism therefore remain.